scientific speculations part II: our solar system

As I'm sure you realize, our solar system consists of four
small, "rocky" planets orbiting relatively close to the sun, and
four "gas giant" planets orbiting at a much greater distance away
(Pluto is no longer considered a planet). Of all of the planets,
there is only one where we are certain life exists, and that, of
course, is Earth. But why is that? Both Venus and Mars are not
all that different in size from Earth, and when compared to the
orbital paths of the "gas giants", are at a relatively similar
distance from the sun. So why did life evolve and flourish on
our planet, but not on our neighbors? Well, consider the fact
that the animals and plants on Earth are all primarily composed
of water, and, in fact, wouldn't be able to live at all without
it. It is a miraculous substance, and life thrives in it.

As everyone knows, water exists in three states; ice, liquid
water, and water vapor. On Earth, all exist, although liquid
water clearly predominates. And it is liquid water that is
conspicuously missing from from Venus and Mars. While Venus is
too hot for liquid water to exist (sometimes reaching 900F), Mars
is too cold, with it's water trapped in polar ice caps. For life
to exist on a planet in our solar system then, it would have to
orbit somewhere between Venus and Mars for the temperature to
allow the formation of liquid water. The question to be
pondered, I think, is whether Earth is orbiting in just about the
only path where life could have evolved, or whether Earth's
potential orbit could have been anywhere in a large band
somewhere between Venus and Mars. Of these two planets, Venus
seems to be the most inhospitable, with it's crushing, corrosive
atmosphere and 900F temperature, and yet, according to
scientists, the vast majority of this incredibly high temperature
can be attributed to a run away green house effect, and not
solely to the fact that it is closer to the sun. As far as Mars
is concerned, telescopic evidence clearly shows that liquid water
once flowed on it's surface. And so, I think it likely that a
wide band, of unknown width, exists in out solar system where,
should a planet be lucky enough to orbit within it, life could
potentially evolve. Earth just happened to be orbiting within
that band.